ChannelSPURIOUS EMISSIONS
Sheet 2 · All episodes

Episode 3 · AIS · Series 01: The Listening Station

How do I see ships?

A frame from the draft of episode 3, AIS
Release
Length
5:29
On the dial
162 MHz
Signal
AIS

What it covers

Every large ship broadcasts its position on marine VHF radio. In this video I explain how AIS works, from the time slots the ships share to the 168 bits of a position report, and how my own receiver decodes it and shares it with five tracking networks.

The written guide: set this up yourself

Chapters

  1. 0:00Intro
  2. 0:31The broadcast
  3. 1:10Taking turns
  4. 1:42The signal
  5. 2:40My receiver
  6. 3:23The message
  7. 3:52My station
  8. 4:36The result

Frames from the draft

AIS: Intro
01 Intro
AIS: The broadcast
02 The broadcast
AIS: Taking turns
03 Taking turns
AIS: The signal
04 The signal
AIS: My receiver
05 My receiver
AIS: The message
06 The message
AIS: My station
07 My station
AIS: The result
08 The result

Gear in this episode

AIS-band vertical antennathe antenna on my roof for ships
Nooelec Flamingo FMthe filter that blocks FM broadcast stations
Airspy Minithe USB radio I use for AIS
Awxlumv aluminum heat sinks, 50 x 25 x 10 mm (2 pack)the heat sinks I add to most of my SDR dongles (not part of the radios as sold)
Raspberry Pi 5the small computer that runs the decoder
Rosonway RSH-A10 powered USB hubfeeds power and data to the radios on Donatello
CyberPower CP1500PFCLCD UPS (1500 VA, sine wave)the battery backup that keeps almost all of my station running through a power cut

Software and references

AIS-catchergithub.comthe decoder I run (open source)
docker-shipfeedergithub.comAIS-catcher packaged with feeders for the tracking networks
AIVDM/AIVDO protocol decodinggpsd.gitlab.iothe best plain-language reference for AIS message formats

What is simplified, and what is invented

  • The slot picture is simplified: real ships leave many more slots between their reports.
  • The frequency-shift picture is simplified: AIS uses GMSK modulation with NRZI coding.
  • The example message is rebuilt from values my station decoded for a real ferry.
  • Map data © OpenStreetMap contributors (openstreetmap.org/copyright).